Water and Solute Permeability of Plant Cuticles: Measurement and ...

4.6 Water Permeability of Isolated Astomatous Cuticular Membranes 101
of the radioactivity in the wax (100Bqmg −1 ) and the partition coefficient Kww
(5.5 × 10 −5 ) is 1.82 × 10 6 Bqmg −1 or 1.82 × 10 9 Bqg −1 water. Transfer from the
vapour phase into the scintillation cocktail requires special equipment to prevent
desorption of water from the thin wax film. Since wax platelets cannot be handled
without support (for instance aluminium foil), water sorption must be measured with
different wax amounts, to be able to correct for sorption on the support.
4.6.2.3 Co-Permeation of Water and Lipophilic Solutes
Co-permeability experiments of lipophilic solutes and water were conducted to
identify the diffusion paths of water and lipophilic solutes in the CM (Niederl
et al. 1998). The cuticle is a lipophilic polymer membrane (Chap. 1), and permeability
of lipophilic non-electrolytes increases with increasing partition coefficients
(Sect. 6.2). This is due to the fact that the concentration of solutes in CM and
wax increase with increasing K. Using 3 H-labelled water (THO) and 14 C-labelled
organic solutes (benzoic acid, salicylic acid), simultaneous penetration of each of
the two pairs of THO and 14 C-labelled organic solutes across Prunus laurocerasus
CM was measured (Fig. 4.16). Donor solutions were buffered at pH 3, and the concentration
of non-ionised solutes was used as driving force. The receiver contained a
phospholipid suspension (1%) in which lipophilic solutes are sorbed such that their
concentration in water is essentially zero.
Flow of water (mol/m 2 )
0.012
0.010
0.008
0.006
0.004
0.002
benzoic acid (slope 244)
salicylic acid (slope 83)
0.000
0 1e-5 2e-5 3e-5 4e-5 5e-5
Flow of solutes (mol/m 2 )
Fig. 4.16 Simultaneous penetration of 3 H-labelled water and 14 C-labelled benzoic acid and salicylic
acid across isolated cuticular membranes of Prunus laurocerasus. Data points represent the
amounts of labelled compounds which had diffused across each of the 15 investigated cuticles at
each sampling time (1, 2, 3, 4, 5 and 6 h). (Data from Niederl et al. 1998)